How do you do a somersault in the water?

Hello, I'm new here. Finally gave in and decided to turn to the internet for help. I don't even know if I'm supposed to post this here or not, please delete/move it if it's in the wrong place. I'm a bit embarrassed as this is going to seem like a stupid question to most of you...but how exactly do you do an underwater somersault? Yes, I'm 27 and never even learned. I've been taking swimming lessons over the past few years, trying to get caught up. My front crawl and breaststroke, while no where near the skill level of most of you, have nonetheless improved greatly, which I am happy about. For the past few weeks, my class has started to learn the flip turn. However, I cannot even do a basic somersault. Everyone else in the class picked it up instantly, no problems whatsoever. But me, I'm the only one in the class who can't even do a simple underwater somersault. I've been practicing over and over, but no luck. Is it something that you either can or can't do? I've heard that some people can't float no matter what...is that also true for somersaults? I'm at my wits end and am really getting frustrated with this, after all this practicing it seems like I should be seeing some results by now. Is there some kind of drill or routine I can do with a flotation device to help flip under the water? Is there a shortcut or way to break the motion down into parts? I can't seem to make my body go all the way around, instead, I get pulled back up by the water and end up looking like an idiot. My teacher is great but she can't help me with the flip, she's got a full class to teach and so I basically have been told to work this out on my own. Which I've been trying to do for the past few weeks but have failed as of yet. Well. At this point any advice at all would be helpful, thanks in advance. Sorry if I seem angry, I've just about had it, weeks of practice and no luck.